1. Field of the Invention
The invention concerns a system for the optical pumping of a cell of atomic or molecular gases and its application to a magnetometer.
2. Description of the Prior Art
The excitation of cells of atomic or molecular gases such as helium, rubidium or cesium, using a monochromatic light beam, is known and has been the object of a great many applications in many devices such as magnetometers.
A description of a magnetometer using the nuclear magnetic resonance effect will be found in the article: "A Nuclear Free Precession Magnetometer using Optically Polarized He.sup.3 Gas" by R.E. Slocum and B.I. Marton in IEEE, April, 1974.
The use of the optical pumping properties of helium 3 and helium 4 is now well established in the detection of weak magnetic fields. In the case of helium 4, an electrical discharge is used to create atoms in the metastable state 2S. Using a suitably polarized light, resonant towards 1.08 .mu.m, one or more components of the transition 2S-2P are excited from this metastable state. Thus, by optical pumping, the metastable atoms are oriented or aligned in the state 2 S. When helium 3, which has a non-null nuclear spin, is used, the nuclear spin is thus oriented in the fundamental state through the hyperfine electron-nucleus interaction and the collisions exchanging the speed between atoms in the metastable state and atoms in the fundamental state.
The helium 4 magnetometer is based on the measurement of electron magnetic resonance in the magnetic field to be measured. The resonance frequency gives the value of the field to be measured. The electron resonance is detected by the variation in the absorption coefficient induced on a beam of circularly polarized light at 1.0 .mu.m or by the variation in the polarization of fluorescence light.
The helium 3 magnetometer is based on the measurement of the nuclear magnetic resonance or the free precession of the nucleii polarized in the fundamental state. In the fundamental state, the nuclear relaxation time may be very long, greater than 24 hours, if a suitable gas cell is used. This enables separating the operations for orienting the nuclear spin and for measuring the precession frequency of these oriented spins. The measuring operation can thus be performed continuously, or repeated at regular intervals without it being necessary to orient the nucleii.
These magnetometers work with optimum values of helium pressure of the order of 1 torr. It can be shown that the optical power needed to obtain high rates of orientation or electron or nuclear alignment is of the order of some hundreds of milliwatts in the usual geometries. The use of lasers matched to one of the transition frequencies therefore calls for relatively high power, especially if semiconductor lasers are considered as sources. Furthermore, these lasers have wide lines compared to the helium lines when they are not monomode.
An object of the present invention is a method to increase the efficiency of the pumping of atomic or molecular gas cells and notably, of helium cells, by a laser diode.